US4707602A - Fourier transform time of flight mass spectrometer - Google Patents
Fourier transform time of flight mass spectrometer Download PDFInfo
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- US4707602A US4707602A US06/801,207 US80120785A US4707602A US 4707602 A US4707602 A US 4707602A US 80120785 A US80120785 A US 80120785A US 4707602 A US4707602 A US 4707602A
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
- H01J49/00—Particle spectrometers or separator tubes
- H01J49/02—Details
- H01J49/022—Circuit arrangements, e.g. for generating deviation currents or voltages ; Components associated with high voltage supply
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
- G01N27/62—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating the ionisation of gases, e.g. aerosols; by investigating electric discharges, e.g. emission of cathode
- G01N27/622—Ion mobility spectrometry
- G01N27/623—Ion mobility spectrometry combined with mass spectrometry
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
- H01J49/00—Particle spectrometers or separator tubes
- H01J49/26—Mass spectrometers or separator tubes
- H01J49/34—Dynamic spectrometers
- H01J49/40—Time-of-flight spectrometers
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
- H01J49/00—Particle spectrometers or separator tubes
- H01J49/26—Mass spectrometers or separator tubes
- H01J49/34—Dynamic spectrometers
- H01J49/40—Time-of-flight spectrometers
- H01J49/403—Time-of-flight spectrometers characterised by the acceleration optics and/or the extraction fields
Definitions
- the present invention relates in general to time of flight mass spectrometers, and, more particularly, to such a spectrometer operated in such a manner as to derive an output in the frequency domain which is thence Fourier transformed to the time domain to obtain a conventional time of flight mass spectrometer output.
- time of flight mass spectrometers have employed a first gate between the ion source and the drift tube and a second gate operatively associated with the detector at the output of the drift tube. These gates were driven by the same modulation function which was a pseudo random pulse sequence with the second gate signal being variably delayed relative to the first gate signal. The variable delay was then swept over a range to obtain an output mass spectrum in the time domain, i.e., a conventional time of flight mass spectrum.
- the advantage of utilizing the two gates driven with the same modulation function is that the duty cycle, i.e., the number of ions passing between the source and the detector and signal-to-noise ratio are greatly increased over the more conventional time of flight mass spectrometer in which ions from the source were gated with a short pulse and their arrival time measured at the detector for deriving a mass spectrum of the sample under analysis.
- the more conventional time of flight mass spectrometer is disclosed in U.S. Pat. No. 4,458,149 issued Jul. 3 1984.
- the pseudo random gated time of flight mass spectrometer is disclosed in an article entitled: "A Miniature Time of Flight Mass Spectrometer" appearing in Vacuum, volume 21, #10, pgs. 461-464 published by Pergamon Press in 1971.
- pseudo random modulated time of flight mass spectrometer provides increased duty cycle and thus signal-to-noise as contrasted with the more conventional time of flight mass spectrometer
- the associated random pulse generator, variable delay, and associated amplifiers are relatively sophisticated and costly due to requirements for relatively broad band phase linear performance.
- the principal object of the present invention is the provision of an improved time of flight mass spectrometer employing Fourier transform signal processing.
- the flow of ions from the ion source through the drift tube region to the detector is modulated at a certain modulation frequency which is swept and the output is detected as a function of the modulation frequency to obtain an ion mass interferogram output which is thence Fourier transformed into the time domain to obtain a time of flight mass spectrum of the substance under analysis, whereby improved signal-to-noise is obtained with a relatively high duty cycle while utilizing less costly signal measurement electronics.
- the flow of ions from the drift region into the detector is modulated at the same modulation frequency at which the flow of ions from the source into the drift region is modulated.
- the flow of ions from the source into the drift region and the flow of ions from the drift region into the detector are both modulated simultaneously with the same modulation frequency, thereby simplifying the ion flow modulation scheme.
- the flow of ions from the source into the drift region is modulated with a sinusoidal function at the modulation frequency.
- FIG. 1 is a schematic diagram, partly in block diagram form, of a Fourier transform mass spectrometer incorporating features of the present invention
- FIG. 2 is a plot of various waveforms encountered in the system of FIG. 1;
- FIG. 3 is an interferogram output derived at the output of the detector in the system of FIG. 1;
- FIG. 4 is a time of flight mass spectrum of a sample of toluene as derived from the output of the spectrometer of FIG. 1.
- the spectrometer 11 includes an evacuable envelope 12 containing certain elements of the spectrometer.
- the envelope 12 and its internal elements are substantially the same as those of the conventional time of flight spectrometer having a one meter length flight tube, for example, the model 2001 time of flight mass spectrometer commercially available from CVC Products of Rochester, N.Y.
- the evacuable envelope 12 includes an ion source 13, at one end, for ionizing a substance under analysis admitted into the envelope 12 by means of tubulation 14.
- ion source 13 Operatively associated with the ion source 13 are draw-out and acceleration grids 15 and 16, respectively, for drawing out the ions from the ion source 13 and accelerating them into a field-free drift region 17 to an ion detector 18 at the far end of the envelope 12.
- Electrostatic steering plates 19 of conventional design are disposed at the entrance to the drift region 17 for steering the ions into and through the drift region to the detector 18.
- the ion source 13 includes a thermionic emitting filament 21 for directing a stream of electrons across the ion source to an electron collector 22.
- the electron current in the electron beam serves to ionize the gaseous species under analysis.
- a backplate 23 of the ion source 13 serves as a repeller electrode and in cooperation with the first accelerating or draw-out grid 15 serves to draw the ionized gaseous constituents into the accelerating grid assembly 16 and thence through the steering plates 19 into the drift region 17.
- the ion detector 18 includes an electron multiplier section 25, which multiplies the electron secondary emission generated by impingement of an ion on a collector electrode portion 26 of the electron multiplier 25.
- the multiplied electron current is collected on an anode 27 of the detector 18 and a detector electrode 28 is provided for gating on and off the electron current to the anode 27.
- the spectrometer 11 includes a frequency generator 31 which produces a sinusoidal output signal at a given frequency. This output is amplified in an rf amplifier 32 and thence fed via d.c. isolating capacitors 33 and 34 to the draw-out electrode 15 and detector gate 28, respectively.
- the ion source 13 and detector 18 are both gated simultaneously with the same modulation function, i.e., sinusoidal waveform produced from the output of the frequency generator 31.
- D.C. biasing voltages are applied to the respective draw-out grid 15 and detector gate electrode 28 via d.c. voltage supplies 35 and 36, respectively.
- the frequency generator 31 comprises a model 200 frequency synthesizer commercially available from Programmed Test Source Inc. of Littletown, Ma. Its output frequency is tunable from 1 to 200 MHz.
- the output frequency of the frequency generator 31 is stepped or tuned over a desired tuning range by means of a microprocessor 38, such as an Apple IIe computer, which is interfaced to the frequency generator 31 by means of a general purpose interface circuitboard 39 such as a model 7490A commercially available from California Computer Systems of Sunnyvale, Calif.
- the microprocessor 38 is operated by means of a conventional keyboard 41.
- the output signal received on anode 27 of the detector 18 is fed to one input of a fourier transform computer 42 such as a model 1080 instrument computer commercially available from Nicolet Instrument Corp. of Madison, Wis.
- the frequency output of the frequency generator 31 is correlated with the respective channels of the fourier transform computer 42 by means of an output derived from the microprocessor 38 and fed to the fourier transform computer 42 via the interface board 39.
- the output signal of the detector 18 is an ion interferogram signal of the type shown in FIG. 3 in which the detected ion current is obtained as a function of the modulation frequency applied to gates 15 and 28.
- This interferogram output signal is fourier transformed from the frequency domain to the time domain utilizing the conventional fourier transform properties of the computer 42 to obtain a conventional time of flight mass spectrum which is thence recorded on the recorder 43.
- the recorded output is as shown in FIG. 4 where the intensity of various mass lines are shown as a function of flight time.
- the spectrum depicted in FIG. 4 is that obtained from a sample of toluene being analyzed.
- the interferogram of FIG. 3 represents 2,048 data points. This interferogram was taken between 1 to 17 MHz. The lower frequency was determined by the range of the frequency generator 31. The upper frequency is the limit of useful information. For this data set, the frequency generator 31 was stepped at 8 KHz per data point and was held at each frequency for one second for a total scan time of 30 minutes. More rapid scans are possible with a more rapidly swept frequency generator 31.
- FIG. 4 shows the intensity spectrum of the fourier transform of the interferogram illustrated in FIG. 3. Only the first 250 data points of the transform are shown and this represents the time domain time of flight mass spectrum for the sample under analysis, namely, toluene.
- the modulation function or waveform is a sine wave, although any periodic function is acceptable.
- a whole family of flight times are selected for each applied frequency and the distribution of flight times detected is varied by changing the modulation frequency.
- the distribution of flight times that are selected by a modulation sequence can be represented by the correlation of the source transmission function and the signal sampling function. ##EQU1## where t is real time, t f is ion flight time, e(t) is the source transmission modulation function, f(t) if the signal sampling function, T is the integration time of the electronics (essentially infinity here).
- a value of ⁇ (t f ) for a particular set of modulation conditions represents the fraction of ions of flight time t f that reach the anode 27 and get measured. If a sample having TOF spectrum m(t f ) is measured, the detected signal is
- Equation (1) the intrinsic intensity of ions of flight time t f , m(t f ), is multiplied by the fraction of those ions that get detected, ⁇ (t f ), and that product is summed over all flight times.
- S and ⁇ (t f ) are functions of the particular modulation sequence through Equation (1).
- e(t) equals f(t) and will be represented as a binary square wave.
- FIG. 2a shows the form of e(t) used.
- FIG. (2b) is a triangle wave--the autocorrelation function of FIG. (2a).
- ⁇ (t f , ⁇ ) illustrates the relative efficiency of detection of ions of flight time t f for a modulation frequency ⁇ . Ions whose flight times are integral values of the reciprocal of the applied modulation frequency are detected with maximum efficiency. Ions with flight times equal to half integral values of the reciprocal of the modulation frequency are not detected at all; and ions with flight times intermediate to integral and half integral values of the frequency are detected with intermediate efficiency.
- the interferogram output signal is obtained as a function of frequency.
- the value of the output waveform is given by
- the interferogram output signal, S( ⁇ ) and the TOF spectrum, m(t f ), are ⁇ --transforms of each other, and ⁇ (t f , ⁇ ) is the kernel of the transform. It is the periodic and essentially cosinusoidal nature of ⁇ (t f , ⁇ ) which will justify Fourier transformation. In fact, if a sinusoidal modulation function, had been used rather than the square wave, ⁇ (t f , ⁇ ) would be cos 2 ⁇ t and S( ⁇ ) would be the cosine transform of m(t f ).
- FIG. (2c) illustrates that the TOF spectrum consists of a single component of a single flight time.
- This single ion interferogram is illustrated in FIG. (2c).
- the detector signal is a maximum.
- the applied frequency is equal to a half integral value of the ion flight time, the detector signal is zero.
- FIG. (2b and c) illustrate that ⁇ (t f , ⁇ ) is a periodic function of two variables.
- the recorded interferogram as a function of frequency for the rapid scan case has a flight time dependent phase shift of 0.5 Rt f ,.
- the period of the interferogram is still 1/t f .
- the magnitude spectra are unaffected by the scan rate. For this reason the magnitude spectra of the interferograms is calculated.
- Sensitivity, or S/N is related to the amount of time that the source is broadcasting to the detector, or in other words, the duty cycle; while the resolution is proportional to the modulation bandwidth.
- bandwidth is achieved by modulating the source with a very narrow function, the narrower the pulse width, the higher the bandwidth of the modulation and the higher the resolution. This bandwidth is achieved at the expense of the duty cycle of the instrument, though.
- frequency bandwidth is achieved by measuring the interferogram over a large range of modulation frequencies while maintaining a 25% duty cycle throughout.
- the mathematical transform utilized has been the Fourier transform, for transforming the frequency domain interferogram into the time domain spectrum
- other mathematical transformations may be used. More particularly, some applicable conventional mathematical techniques are the Walsh transform or the LaPlace transform.
- the advantage of the Fourier transform time of flight mass spectrometer of the present invention is that it results in higher signal strengths compared to conventional time of flight operation and this is translatable to increased sensitivity and decreased scan time for the time of flight mass spectrometer.
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Abstract
Description
S=∫m(t.sub.f)ε(t.sub.f) dt.sub.f Eq. (2)
S (ν)=∫m(t.sub.f)ε(t.sub.f, ν) d t.sub.f Eq. (3)
ε(t.sub.f,t)=cos (ν.sub.o t.sub.f +Rtt.sub.f -1/2Rt.sub.f.sup.2) Eq. (8)
ε(t.sub.f,ν)=cos 2π(ν-1/2Rt.sub.f)t.sub.f Eq. (9)
Claims (8)
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US06/801,207 US4707602A (en) | 1985-04-08 | 1985-11-25 | Fourier transform time of flight mass spectrometer |
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US06/724,514 US4633083A (en) | 1985-04-08 | 1985-04-08 | Chemical analysis by time dispersive ion spectrometry |
US06/801,207 US4707602A (en) | 1985-04-08 | 1985-11-25 | Fourier transform time of flight mass spectrometer |
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US06/724,514 Continuation-In-Part US4633083A (en) | 1985-04-08 | 1985-04-08 | Chemical analysis by time dispersive ion spectrometry |
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Cited By (23)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
GB2237882A (en) * | 1989-10-13 | 1991-05-15 | Southwest Sciences Inc | Mass spectroscopic apparatus and method |
GB2300296A (en) * | 1995-04-26 | 1996-10-30 | Bruker Franzen Analytik Gmbh | A method for measuring the mobility spectra of ions with ion mobility spectrometers(IMS) |
WO1998008244A2 (en) * | 1996-08-17 | 1998-02-26 | Millbrook Instruments Limited | Charged particle velocity analyser |
US5789745A (en) * | 1997-10-28 | 1998-08-04 | Sandia Corporation | Ion mobility spectrometer using frequency-domain separation |
US5900628A (en) * | 1996-04-03 | 1999-05-04 | Jeol Ltd. | Method of processing mass spectrum |
US6300626B1 (en) * | 1998-08-17 | 2001-10-09 | Board Of Trustees Of The Leland Stanford Junior University | Time-of-flight mass spectrometer and ion analysis |
US20020185593A1 (en) * | 2001-04-26 | 2002-12-12 | Bruker Saxonia Analytik Gmbh | Ion mobility spectrometer with non-radioactive ion source |
US6580068B1 (en) * | 1999-07-09 | 2003-06-17 | Sandia Corporation | Method and apparatus for time dispersive spectroscopy |
WO2003096374A1 (en) * | 2002-05-08 | 2003-11-20 | Hars Gyoergy | Time-of-flight mass spectrometer with an ion source emitting continuously |
WO2004097394A1 (en) * | 2003-04-30 | 2004-11-11 | Smiths Group Plc | Pseudo-random binary sequence gate-switching for spectrometers |
WO2004102178A1 (en) * | 2003-05-17 | 2004-11-25 | Smiths Group Plc | Spectrometer systems |
US20050045817A1 (en) * | 2003-09-03 | 2005-03-03 | Shinichi Yamaguchi | Time of flight mass spectrometer |
US6870157B1 (en) * | 2002-05-23 | 2005-03-22 | The Board Of Trustees Of The Leland Stanford Junior University | Time-of-flight mass spectrometer system |
US20060020401A1 (en) * | 2004-07-20 | 2006-01-26 | Charles Stark Draper Laboratory, Inc. | Alignment and autoregressive modeling of analytical sensor data from complex chemical mixtures |
US20080087816A1 (en) * | 2004-07-02 | 2008-04-17 | Mccauley Edward B | Pulsed Ion Source for Quadrupole Mass Spectrometer and Method |
US20090236514A1 (en) * | 2008-03-19 | 2009-09-24 | Uwe Renner | Measurement of ion mobility spectra |
GB2460341A (en) * | 2008-05-30 | 2009-12-02 | Bruker Daltonik Gmbh | Method of Measuring Mobility of Mass Selected Ions |
US20100320375A1 (en) * | 2009-06-22 | 2010-12-23 | Uwe Renner | Measurement of ion mobility spectra with analog modulation |
DE102009048063A1 (en) | 2009-09-30 | 2011-03-31 | Eads Deutschland Gmbh | Ionization method, ion generating device and use thereof in ion mobility spectrometry |
US20140326869A1 (en) * | 2011-10-26 | 2014-11-06 | Tofwerk Ag | Method and apparatus for determining a mobility of ions |
US20160013039A1 (en) * | 2013-03-05 | 2016-01-14 | Micromass Uk Limited | Spatially Correlated Dynamic Focusing |
GB2553863A (en) * | 2016-09-20 | 2018-03-21 | Micromass Ltd | Improved method of ion mobility spectrometry |
US10684255B2 (en) | 2015-03-24 | 2020-06-16 | Micromass Uk Limited | Method of FT-IMS using frequency modulation |
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Cited By (50)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
GB2237882B (en) * | 1989-10-13 | 1994-04-06 | Southwest Sciences Inc | Mass spectroscopic apparatus and method |
GB2237882A (en) * | 1989-10-13 | 1991-05-15 | Southwest Sciences Inc | Mass spectroscopic apparatus and method |
GB2300296A (en) * | 1995-04-26 | 1996-10-30 | Bruker Franzen Analytik Gmbh | A method for measuring the mobility spectra of ions with ion mobility spectrometers(IMS) |
US5719392A (en) * | 1995-04-26 | 1998-02-17 | Bruker Saxonia Analytik Gmbh | Method of measuring ion mobility spectra |
GB2300296B (en) * | 1995-04-26 | 1999-06-09 | Bruker Franzen Analytik Gmbh | Method of measuring ion mobility spectra |
US5900628A (en) * | 1996-04-03 | 1999-05-04 | Jeol Ltd. | Method of processing mass spectrum |
WO1998008244A2 (en) * | 1996-08-17 | 1998-02-26 | Millbrook Instruments Limited | Charged particle velocity analyser |
WO1998008244A3 (en) * | 1996-08-17 | 1998-04-09 | Millbrook Instr Limited | Charged particle velocity analyser |
US5789745A (en) * | 1997-10-28 | 1998-08-04 | Sandia Corporation | Ion mobility spectrometer using frequency-domain separation |
US6300626B1 (en) * | 1998-08-17 | 2001-10-09 | Board Of Trustees Of The Leland Stanford Junior University | Time-of-flight mass spectrometer and ion analysis |
US6580068B1 (en) * | 1999-07-09 | 2003-06-17 | Sandia Corporation | Method and apparatus for time dispersive spectroscopy |
US20020185593A1 (en) * | 2001-04-26 | 2002-12-12 | Bruker Saxonia Analytik Gmbh | Ion mobility spectrometer with non-radioactive ion source |
US6586729B2 (en) * | 2001-04-26 | 2003-07-01 | Bruker Saxonia Analytik Gmbh | Ion mobility spectrometer with non-radioactive ion source |
WO2003096374A1 (en) * | 2002-05-08 | 2003-11-20 | Hars Gyoergy | Time-of-flight mass spectrometer with an ion source emitting continuously |
US6870157B1 (en) * | 2002-05-23 | 2005-03-22 | The Board Of Trustees Of The Leland Stanford Junior University | Time-of-flight mass spectrometer system |
WO2004097394A1 (en) * | 2003-04-30 | 2004-11-11 | Smiths Group Plc | Pseudo-random binary sequence gate-switching for spectrometers |
US20060273253A1 (en) * | 2003-04-30 | 2006-12-07 | Fitzgerald John P | Pseudo-random binary sequence gate-switching for spectrometers |
WO2004102178A1 (en) * | 2003-05-17 | 2004-11-25 | Smiths Group Plc | Spectrometer systems |
US20050045817A1 (en) * | 2003-09-03 | 2005-03-03 | Shinichi Yamaguchi | Time of flight mass spectrometer |
US7148473B2 (en) * | 2003-09-03 | 2006-12-12 | Shimadzu Corporation | Time of flight mass spectrometer |
US20080087816A1 (en) * | 2004-07-02 | 2008-04-17 | Mccauley Edward B | Pulsed Ion Source for Quadrupole Mass Spectrometer and Method |
US7507954B2 (en) * | 2004-07-02 | 2009-03-24 | Thermo Finnigan Llc | Pulsed ion source for quadrupole mass spectrometer method |
US20060020401A1 (en) * | 2004-07-20 | 2006-01-26 | Charles Stark Draper Laboratory, Inc. | Alignment and autoregressive modeling of analytical sensor data from complex chemical mixtures |
DE102008015000A1 (en) | 2008-03-19 | 2009-10-08 | Bruker Daltonik Gmbh | Method for measuring ion mobility spectra |
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US20090236514A1 (en) * | 2008-03-19 | 2009-09-24 | Uwe Renner | Measurement of ion mobility spectra |
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GB2460341A (en) * | 2008-05-30 | 2009-12-02 | Bruker Daltonik Gmbh | Method of Measuring Mobility of Mass Selected Ions |
US20090294647A1 (en) * | 2008-05-30 | 2009-12-03 | Bruker Daltonik Gmbh | Measuring the mobility of mass selected ions |
DE102008025972A1 (en) * | 2008-05-30 | 2009-12-31 | Bruker Daltonik Gmbh | Method for measuring the mobility of mass spectrometrically selected ion species |
GB2460341B (en) * | 2008-05-30 | 2013-04-03 | Bruker Daltonik Gmbh | Method of measuring mobility of mass selected ions |
US8022359B2 (en) * | 2008-05-30 | 2011-09-20 | Bruker Daltonik Gmbh | Measuring the mobility of mass selected ions |
US8198584B2 (en) * | 2009-06-22 | 2012-06-12 | Bruker Daltonik Gmbh | Measurement of ion mobility spectra with analog modulation |
US20100320375A1 (en) * | 2009-06-22 | 2010-12-23 | Uwe Renner | Measurement of ion mobility spectra with analog modulation |
WO2011039010A3 (en) * | 2009-09-30 | 2011-06-03 | Eads Deutschland Gmbh | Ionization method, ion source and uses of the same in ion mobility spectrometry |
WO2011039010A2 (en) | 2009-09-30 | 2011-04-07 | Eads Deutschland Gmbh | Ionization method, ion producing device and uses of the same in ion mobility spectrometry |
DE102009048063A1 (en) | 2009-09-30 | 2011-03-31 | Eads Deutschland Gmbh | Ionization method, ion generating device and use thereof in ion mobility spectrometry |
US8987681B2 (en) | 2009-09-30 | 2015-03-24 | Eads Deutschland Gmbh | Ionization method, ion producing device and uses of the same in ion mobility spectrometry |
US20140326869A1 (en) * | 2011-10-26 | 2014-11-06 | Tofwerk Ag | Method and apparatus for determining a mobility of ions |
US9366650B2 (en) * | 2011-10-26 | 2016-06-14 | Tofwerk Ag | Method and apparatus for determining a mobility of ions |
US9671369B2 (en) | 2011-10-26 | 2017-06-06 | Tofwerk Ag | Method and apparatus for determining a mobility of ions |
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GB2553863B (en) * | 2016-09-20 | 2021-12-08 | Micromass Ltd | Improved method of ion mobility spectrometry |
GB2597391A (en) * | 2016-09-20 | 2022-01-26 | Micromass Ltd | Improved method of ion mobility spectrometry |
GB2597391B (en) * | 2016-09-20 | 2022-07-06 | Micromass Ltd | Improved method of ion mobility spectrometry |
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